1. Field of the Invention
The present invention relates to a communications system and, more particularly, to a communications system in which a transmission mode (modulating and coding ratio) in a transmitter apparatus is proposed by a receiver apparatus and a processing method therefor, and a computer program for having a computer execute this method.
2. Description of the Related Art
With communications apparatuses based on wireless LAN (Local Area Network) and mobile communication, for example, sol-called adaptive transmission is proposed in which transmission modes are dynamically changed over in accordance with propagation path situations so as to enhance transmission efficiency. In this adaptive transmission, transmission modes, such as modulation schemes and coding ratios, change in accordance with propagation path situations. If the MIMO (Multiple Input Multiple Output) scheme is applied, the number of streams is also included in the transmission modes.
The methods of determining a transmission mode in the adaptive transmission is largely classified into (1) a method of determination on the sender side and (2) a method of determination on the receiver side. In the former, the receiver side feeds back channel quality to the sender side. In this case, generally, the quantity of information to be fed back is relatively large, so that transmission efficiency is low. In the latter, only a finite number of transmission modes may be specified on the basis of the propagation path characteristic observed at the receiver side, so that the quantity of information to be fed back is relatively small.
The IEEE (Institute of Electrical and Electronic Engineers) 802.11n standard of wireless LAN and the HSDPA (High Speed Downlink Packet Access) standard of mobile communication use the latter method. However, each transmission mode proposed by the receiver side does not reflect the conditions of the sender side and therefore the sender side does not always use a transmission mode proposed by the receiver side. Namely, each transmission mode fed back from the receiver side provides nothing but a recommended value.
On the other hand, in the selection of a transmission mode by the receiver side, it is a general practice that the quality of a communications path is observed and, of the transmission modes satisfying a certain error ratio, the one having the highest data transfer rate is selected. However, processing delay, change of propagation path characteristic, estimated error, and deficiency of observable channel information, for example, do not make complete the selection criteria, thereby requiring the correction of the transmission mode decision criteria by use of a post-event phenomenon, such as a received packet error ratio, for example. For more information, refer to Japanese Patent Laid-open No. 2004-64797 (FIG. 8).